1. Field of the Invention
The present invention relates to a surge-pressure removing system for back-pressure generated in an injection cylinder of a die cast machine when a low-speed injection region is switched over to a high-speed injection region.
2. Description of the Related Art
FIG. 5 shows a conventional example of a surge-pressure removing system in a die cast machine. In this drawing, an accumulator 29 is connected to a port 13 of an injection cylinder 11 through an oil discharge pipe 14, an on-off valve 15 and a branch pipe 18. This accumulator 29 is filled up with nitrogen gas 26, and a piston 28 is constantly biased downwards in the drawing. The on-off valve 15 is provided with pilot circuits 16 and 17. The pilot circuit 16 makes the on-off valve 15 normally closed, and the pilot circuit 17 makes the on-off valve 15 normally open. Two limit switches 19 and 20 are disposed along a moving direction of a dog 21 fixed to a piston rod 12. When the dog 21 is engaged with the limit switch 19 or 20, an injection speed switching signal for the injection cylinder 11 is output from the limit switch 19 or 20. An injection control valve 22 to be actuated in response to this signal controls a flow rate of oil from an accumulator 24 having gas 26 filled through a piston 25 thereby performs the switching control. Incidentally, designated at 23 is an oil pressure source and 27 an oil pressure tank.
With the above-described arrangement, the pressure of nitrogen gas 26 in the accumulator 29 is set by measuring the lowest surge-pressure conditions by trial operations of the die cast machine. The total sum of the fill-up pressure of nitrogen gas 26, the gravity of the piston 28, the sliding resistance and the like constantly act on the piston 28 of the accumulator 29 as a downward-directed force. However, when surge-pressure is produced in the oil discharge pipe 14, the piston 28 moves upwardly. Accordingly, when the oil pressure in the oil discharge pipe 14 abruptly rises in accordance with a change in pressure at the time the injection cylinder 11 is switched over to the high speed injection region from the low speed injection region, the oil is abruptly supplied into the accumulator 29 and, in accordance with the movement of the piston 28, nitrogen gas 26 in the accumulator 29 is adiabatically compressed, so that the surge-pressure generated in the oil discharge pipe 14 can be controlled low.
However, in the conventional surge-pressure removing system, it was a limit to reduce the surge-pressure to 70-180kg/cm.sup.2. The reason seems to reside in that, since the piston in the accumulator 29 is pressurized by nitrogen gas, the piston 28 cannot instantaneously respond even if an abrupt change in pressure occurs.